FR: Override default diff/merge editor without resorting to --config-toml "ui.{diff,merge}-editor=…"

[amiryal]

I like having the full comfort of meld-3 with the mouse and a GUI, but sometimes I need to pick just a small thing and stay in the terminal with both hands on the keyboard. For all the commands that refer to ui.diff.tool in the configuration, we already have the --tool option to override it, e.g. with --tool :builtin to stay in the terminal.

The feature request is to add similar --tool options for overriding ui.diff-editor and ui.merge-editor (as appropriate) to all of the commands that refer to them.

[yuja]

Duplicates of #2575. For now, you could use command [aliases]